1. Field of the Invention
The invention relates in general to electrical inductive apparatus, such as transformers and electrical reactors, and more specifically to such apparatus in which at least one of the windings is formed of electrically conductive sheet, strip or foil material.
2. Description of the Prior Art
In electrical inductive apparatus, such as transformers and electrical reactors, there is a magnetic leakage flux which goes outside of the magnetic core. The insulation space required between the primary and secondary windings of a transformer, for example, destroys the ideal coupling and contributes to the leakage flux. The leakage flux is the difference between the flux of the primary and secondary windings and is predominantly determinative of the reactance of the transformer.
The leakage flux is generated in the windings and is greatest in the cylindrical space between the primary and secondary windings. From the upper and lower part of this channel it must find a return path. A portion of the leakage flux finds its way inwardly against the magnetic core and it utilizes the magnetic core and core parts, such as the bolts and end frames, as a return path. Another portion of the leakage flux finds a return path through the oil and through such transformer parts which have the greatest permeability, for example through the tank walls. The leakage flux produces eddy current losses and hysteresis losses in these components. If one or more of the electrical windings is formed of sheet, strip or foil conductor, the radial component of the leakage flux produces losses in this type of winding. All of these losses combine to reduce not only the efficiency of the transformer, but they may also lead to harmful local heating which may break down and destroy electrical insulation adjacent thereto.
There have been proposed many arrangements for directing the leakage flux to provide a return path for it and avoid losses in the iron parts of the electrical apparatus. It is a known practice (German Pat. No. 366,467) to arrange magnetic shunts or yokes of magnetic laminations on both sides of the windings parallel to the yoke of the magnetic core to provide a low reluctance return path for leakage flux. It is also a known practice to arrange magnetic shunts between such yokes and the magnetic core in order to conduct the leakage fields over to the core (Swiss Pat. No. 365,791). Similarly, it is a known practice to form the pressure plates or parts of the pressure plates with magnetic laminations to conduct the leakage flux inwardly to the magnetic core (German Pat. No. 1,087,262and Norwegian Pat. No. 124,899). It has also been proposed (Electrical Times, March 1971, p. 53) to place magnetic shunts of magnetic laminations on the inside of the windings between the magnetic core and the innermost winding. Such magnetic shunts conduct the leakage flux around the end frames on the core, and thus it is possible to avoid local heating and losses in these components.
In both the primary and secondary windings the leakage fluxes will cause additional losses in the form of eddy current losses. Measurements have shown that the leakage flux in fully symmetrical windings has both an axial and a radial component. For such windings the radial component is about zero in the middle of the winding and it increases toward the axial ends of the winding. In other winding arrangements, for example with windings having different heights, divided windings, separate regulating windings, and the like, the magnitude of the radial flux components across the winding will be dependent upon the winding construction. It is generally well known that it is possible to reduce the additional losses in transformer windings by reducing the dimension of the conductor perpendicular to the flux.
In sheet, strip or foil windings there is a severe additional loss due to radial leakage flux, which limits the use of such windings to specific types of transformer constructions. It is known that it is possible to mount screens of electrically conductive material, especially around the winding ends in the case of sheet, strip or foil windings, in order to reduce the losses (British Pat. No. 990,418). Such screens will set up fields which oppose the leakage field, due to currents which are induced therein.